\begin{tikzpicture}
    \begin{axis}[
      %width=0.8\textwidth,
      %eight=0.6\textwidth,
      axis lines=center,
      axis on top=true,
      xmin=0,
      xmax=32,
      xlabel={Сентябрь},
      xticklabel style={/pgf/number format/1000 sep=},
      ymin=76,
      ymax=80,
      ylabel={Вес (кг)}
    ]
    \addplot coordinates {
      (9,78.5)
      (11,79.4)
      (13,77.9)
      (16,79.0)
      (18,78.8)
      (23,77.2)
      (25,78.2)
      };
    \end{axis}
\end{tikzpicture}